At 4.4 Billion Years Old, Oz Crystals Confirmed As World's OldestZircon crystals found in sandstone on an Australian sheep ranch are so tiny that you'd need a magnifying glass to see them. But recent measurements confirm they offer our earliest glimpse of Earth.

Scientists can show restraint when it comes to superlatives. Think something is the biggest or fastest? Prove it, they say. But it turns out scientists love them because something extreme can help answer interesting questions. NPR's Nell Greenfieldboyce reports on bit of crystal that is so old, it can tell us about some of the Earth's earliest days.

NELL GREENFIELDBOYCE, BYLINE: Our planet's history stretches back more than four and a half billion years. First a spinning cloud of gas and dust coalesced to become our sun. Then clumps of matter started slamming together, forming planets. Scientists think that after a proto-Earth emerged, an object the size of Mars smashed into it. The crash created our moon and turned the Earth into a red ball of molten rock.

JOHN VALLEY: It would have glowed almost like a star. Nothing could exist on the surface. There would be no continental land masses. There'd be no liquid water. And there certainly would have been no life at that time.

GREENFIELDBOYCE: John Valley is a geochemist at the University of Wisconsin-Madison. He wonders: How long did those fiery conditions last? When did land appear, when did our planet become homey? Valley thinks it happened pretty quickly. And his evidence lies inside ancient sand grains that were once part of an early continent.

GREENFIELDBOYCE: Geologists found them on a sheep ranch in a remote part of Australia in sandstones that, three billion years ago, were a beach.

VALLEY: It's always blown my mind, I mean to collect samples that were on a beach three billion years ago and to find crystals that were more than a billion years older even than the beach is just really surprising and wonderful.

GREENFIELDBOYCE: Back in 2001, Valley and some colleagues reported that one of these grains was around 4.4 billion years old, so old that not everyone believed it.

VALLEY: There have been challenges because nothing in science goes without being questioned; it always has to be proven.

GREENFIELDBOYCE: Valley and his colleagues had determined the crystal's age by analyzing how much of the element uranium had decayed into lead. They did this by looking at a small part of the crystal. But some scientists pointed out a problem - lead atoms might move around inside the crystal.

VALLEY: And if that happens, in the places where it's concentrated, you will measure an older age than the true age.

GREENFIELDBOYCE: So Valley's team recently tackled this issue with a powerful new instrument that can pluck out and identify individual atoms. They found that, yes, lead does move around but not enough to affect their age calculations. So they do have the oldest known material of any kind formed on Earth. Sam Bowring is a geologist at MIT. He says it's pretty hard to argue with this new study, which appears in the journal Nature Geoscience.

SAM BOWRING: I think people will be impressed with the technique and impressed with the conclusions and agree with them.

GREENFIELDBOYCE: He says it means continents must have formed just tens of millions of years after the big impact that created the moon.

BOWRING: I think that really is profound, if you think about it, and now we're talking about a history on this planet that goes back to almost the day that the planet was born.

GREENFIELDBOYCE: He says if geologists are lucky, someday they'll stumble over a big rock as old as these little crystals. Nell Greenfieldboyce, NPR News.

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